A polymeric porous material having a large number of pores intercommunicable with each other is utilized in various fields. For example, the polymeric porous material is utilized to produce ultrapure water, purify chemicals, and as a separation film to be used for water treatment, a waterproof breathable film to be used for clothes and sanitary materials, and a separator of a battery.
A secondary battery is widely used as the power source of OA, FA, household appliances, and portable devices such as communication instruments. A lithium-ion secondary battery has a favorable volumetric efficiency when it is mounted on apparatuses and allows the apparatuses to be compact and lightweight. Therefore there is an increase in the use of portable devices in which the lithium-ion secondary battery is mounted.
Owing to research and development of a large secondary battery which has been made in the field of load leveling, UPS, an electric car, and in many fields relating to the problem of energy and environment, the large secondary battery is allowed to have a large capacity, a high output, a high voltage, and an excellent long-term storage stability. Therefore the lithium-ion secondary battery which is a kind of the nonaqueous electrolyte secondary battery has widely spread in its use.
The lithium-ion secondary battery is so designed that the upper limit of the working voltage thereof is usually 4.1V to 4.2V. Because electrolysis occurs in an aqueous solution at such a high voltage, the aqueous solution cannot be used as an electrolyte. Therefore as an electrolyte capable of withstanding a high voltage, a so-called nonaqueous electrolyte in which an organic solvent is used is adopted.
As a solvent for the nonaqueous electrolyte, an organic solvent having a high permittivity which allows a large number of lithium ions to be present is widely used. Organic carbonate ester such as polypropylene carbonate or ethylene carbonate is mainly used as the organic solvent having a high permittivity. As a supporting electrolyte serving as the ion source of the lithium ion in the solvent, an electrolyte having a high reactivity such as lithium phosphate hexafluoride is used in the solvent by melting it therein.
A separator is interposed between the positive electrode of the lithium-ion secondary battery and its negative electrode to prevent an internal short circuit from occurring. Needless to say, the separator is demanded to have insulating performance as its role. In addition the separator is required to have a porous structure so that it has air permeability to allow the movement of the lithium ion and a function of diffusing and holding the electrolyte. To satisfy these demands, a porous film is used as the separator.
Because batteries having a high capacity are used recently, the degree of importance for the safety of the battery has increased.
A shut-down property (hereinafter referred to as SD property) contributes to the safety of the separator for the battery. The SD property has the function of closing pores when the battery has a high temperature of 100° C. to 140° C., thus cutting ion conduction inside the battery, whereby the temperature inside the battery can be prevented from rising. To use the porous film as the separator for the battery, it is necessary for the porous film to have the SD property.
As another property contributing to the safety of the separator for the battery, a break-down property (hereinafter referred to as BD property) is known. The BD property has a function of preventing the film from being broken and keeping the positive electrode and the negative electrode separated from each other even when generated heat does not drop and the temperature of the battery becomes high (not less than 160° C.). The BD property allows insulation to be maintained even at a high temperature and prevents a wide range of short circuit from occurring between the electrodes, thereby preventing the occurrence of an accident such as firing caused by an abnormal heat generation of the battery. Therefore to use the porous film as the separator for the battery, it is preferable for the porous film to have the BD property. It is also preferable that a break-down temperature (hereinafter referred to as “BD temperature”) is as high as possible.
The “BD temperature” means the lowest temperature of temperatures at which the laminated porous film of the present invention is broken when it is heated by placing it in a frame.
In order for the laminated porous film to obtain excellent BD property, dimensional stability at the time of a temperature rise is one of important properties demanded for a battery. When a battery generates heat abnormally, there is a fear that owing to breakage of the separator caused by heat shrinkage, both poles have a short circuit, which may cause the battery to further generate heat. Thus further improvement for heat resistance is demanded.
In order for the laminated porous film to obtain excellent SD property, it is desirable that the laminated porous film has a proper degree of shrinkage factor at temperatures around 100 degrees. This is contradictory to the dimensional stability. Therefore it is very important to take a balance between the shrinkage factor and the SD property.
To comply with the above-described demands, in Japanese Patent Application Laid-Open No. 2003-103624, there is proposed the porous film having a preferable dimensional stability at 105° C. The porous film is obtained by kneading ultra-high-molecular-weight polyethylene and a solvent to form it into a sheet, stretching it, and extracting the solvent.
In U.S. Pat. No. 3,852,492 (patent document 2), there is disclosed the method of producing the separator for the battery consisting of the polyethylene film and the polypropylene film. With the polyethylene film and the polypropylene film being layered one upon another, the film consisting of the layered polyethylene and polypropylene films are stretched in one axial direction at two stages by changing temperature to make the film porous.
Various methods of obtaining porous films by stretching a polypropylene sheet containing β crystal have been proposed. As the characteristic of the method of producing the porous film, the porous structure is obtained by utilizing the β crystal. To obtain the porous structure by stretching the sheet, it is preferable that the sheet contains a lot of the β crystal before the sheet is stretched. This method is a biaxial stretching method and has a very high productivity as a method of obtaining the porous film.
For example, in U.S. Pat. No. 1,953,202 (patent document 3), there is proposed the method of producing the porous sheet by forming the resin composition composed of polypropylene containing a predetermined amount of the filler and the β crystal nucleating agent into a sheet and stretching the sheet at a specific stretching condition. In U.S. Pat. No. 2,509,030 (patent document 4), there is proposed the micro-porous film, made of very transparent polypropylene, which is obtained by biaxially stretching the original polypropylene film having a high (K>0.5) β crystal content rate. In U.S. Pat. No. 3,443,934 (patent document 5), there is proposed the method of producing the porous sheet by crystallizing polypropylene containing a particular amide compound in a specific condition to obtain the solidified material and stretching the solidified material.